The invention concerns an MPI method for localizing magnetic particles within a test sample, wherein a location-dependent magnetic field is applied which has a field-free region, the method comprising the following steps:                determining a calibration volume and a measurement volume, wherein the calibration volume is larger than the measurement volume and wherein the overall measurement volume is arranged within the calibration volume;        detecting calibration signals Sj and creating a system matrix S from the calibration signals Sj;        recording an MPI measuring signal u (MPI time signal or MPI frequency spectrum obtained through Fourier transformation of the MPI time signal), wherein through application of the magnetic control field, the field-free region is moved through the measurement volume by means of a measuring sequence;        reconstruction of a location-dependent magnetic particle concentration with magnetic particle concentration values ci within the calibration volume from the recorded MPI measurement signal u and the created system matrix S and association of the magnetic particle concentration values ci with voxels in the calibration volume.        
A method of this type is disclosed e.g. in [01]-[03].
Magnetic particle imaging (abbreviated as “MPI”) is an imaging method which permits determination of the local distribution of superparamagnetic nanoparticles (in the present case designated as magnetic particles). Towards this end, the magnetic particles are exposed to different static and dynamic magnetic fields in a measurement volume and the magnetization changes of the magnetic particles are detected by means of receiver coils. For spatial encoding in MPI, a magnetic gradient field is applied in the region of the measurement volume which has a field-free region. The field-free region is shifted along a pre-defined trajectory (predetermined dependence of each point of the field-free region) within the measurement volume by means of a dynamic magnetic field (drive field) and/or homogeneous focus fields. By sweeping over the magnetic particles with the field-free region and the associated magnetization change, a measuring signal is generated which is detected by receiver coils.
A system matrix is generated for calibration of the system. For this purpose, a calibration measurement can e.g. be performed, in which a calibration signal is recorded for each measuring point and is stored in the system matrix. The calibration measurement is also performed for positions of the calibration sample outside of the measurement volume, which allows reconstruction of an MPI image of an overall volume which is larger than the measurement volume.
Whereas in [01] and [02] MPI measurements are performed on a phantom with magnetic particles, in which all magnetic particles are located within the measurement volume, [03] describes a method for in vivo recording of a rat heart. During in vivo recordings, magnetic particles are also located outside of the measurement volume. Although these are not swept over by the field-free region, they nevertheless contribute to the measurement signal due to their rotation and unsharpness of the field-free region. This can lead to artefacts in the MPI image.
[04] describes a calibration method for an MPI apparatus, wherein the calibration method comprises m calibration MPI measurements with a calibration sample and generates from these an image reconstruction matrix by means of which the signal contributions of N voxels within a volume under investigation of the MPI apparatus are determined, wherein compressed-sensing-steps are applied using a transformation matrix, which sparsifies the image reconstruction matrix and only a number M<N of calibration MPI measurements for M voxels are performed, from which the image reconstruction matrix is created and stored. In this connection, the volume of the calibration sample may be larger than the volume of one voxel.
It is the underlying purpose of the present invention to propose a method, in particular for in vivo MPI recordings, by means of which low-artefact image data can be generated even with high magnetic particle densities outside of the measurement volume and in a time-saving fashion.